1. Field of the Invention
This invention relates generally to building structures and, more particularly, to load carrying wood frames for supporting floors, ceilings, and the like.
2. Description of the Related Art
Wood structures predominate in residential and light commercial construction. In the case of wood floor framing, the structure typically comprises a series of parallel, spaced-apart wooden spanning members or joists. These joists can be provided in a number of sizes, such as those known in the United States as two-by-twelves, which have standard dimensions of one and one-half inches thick by eleven and one-fourth inches high, and are allowed by most building codes in the United States to have a length of slightly over 21 feet. A range of other sizes is available, including joists known as two-by-eight up through two-by-fourteen. When the span length exceeds 16 feet, vibration and vertical deflection upon loading can become quite noticeable. Even floor systems that are properly constructed and structurally safe can exhibit much movement and resulting noise, which can be quite disconcerting and psychologically troublesome for any occupants.
In the United States, the wood joists are typically installed at a 16-inch spacing, and therefore a typical 20-foot by 20-foot room will require fifteen to sixteen joists. Large 4-foot by 8-foot sheets of plywood are attached to the upper surface of the joists using nails driven through the sheets into the joists, thereby forming the floor subsurface to which flooring, tile, or carpeting can be laid. Blocking, or smaller wood segments that span the distance between joists, is installed below the seams of adjacent plywood sheets to provide a nailing surface.
Loading of the floor, such as when a person walks on the floor, causes vertical deflection and resultant noise and vibration. This occurs because, with 16-inch joist spacing, substantially all of the load is supported by a single joist, which cannot support the pressure sufficiently to eliminate the deflection. Conventionally, the extra vertical stiffness required to stop the deflection is provided by doubling the number of joists, or using an 8-inch joist spacing. This doubles the number of joists needed and therefore doubles the joist costs and dramatically increases the amount of time required for construction.
Even if not present initially, the vibration and vertical deflection upon loading can spontaneously occur, or become worse, with age. This occurs because lumber initially has approximately a 19% moisture content when the structure is erected, and later dries out and stabilizes at approximately a 9% moisture content, thereby shrinking the wood. This can change the relative dimensions of the structure, even pulling against the nails used to erect the structure, causing open spaces to appear and pull the members apart. Thus, when the floors are subject to loading, as when someone walks over them, the members are moved relative to each other, causing vertical deflection and vibration, and often squeaking as well.
Methods other than doubling the number of joists can be used to control the vertical deflection and squeaking of floor systems. Constructing smaller spaces, such as by using joist spans having a length that is less than the maximum allowed by building codes, decreases the amount of vertical deflection. Often, however, this conflicts with the architectural design for the completed structure. If the original span length is to be kept, designers can choose alternate construction materials, such as truss joists, steel beams, and concrete slabs. Unfortunately, these materials are much more costly than comparable wood floor systems.
From the foregoing, it should be apparent that there is a need for a wood floor system that provides reduced vertical deflection upon load and controls vibration and squeaking, that allows for shrinkage of the wood floor structure members, and does so without the high cost and complex assembly of floor systems constructed from materials other than wood. The present invention meets these needs.